a) Field of the Invention
The present invention relates to diagnostic apparatus and method for a motor driven power-assisted steering system of an automotive vehicle.
b) Description of the Related Art
A Japanese Patent Application First Publication No. Heisei 11-342859 published on Dec. 14, 1999 exemplifies a previously proposed diagnostic apparatus for a motor driven power-assisted steering system of an automotive vehicle.
The previously proposed diagnostic apparatus includes a direction abnormality detection circuit to determine whether there is an abnormality in a power-assisted steering direction on the basis of a sub torque signal from a steering torque sensor and a command from a CPU (Central Processing Unit) of a control unit.
The direction abnormality detection circuit determines the abnormality in the power-assisted steering direction depending upon whether a normal/reverse revolution drive command current to command a revolution drive of the motor and a normal/reverse revolution drive command current to command a revolution drive direction.
However, since the actual normal/reverse drive current supplied to the electric motor includes noises, a current which is reverse to the normal/reverse revolution drive command current is instantaneously caused to flow to the electric motor irrespective of the fact that the power-assisted steering direction is normal in a case where the steering torque is small and an absolute value of the normal/reverse drive current is small.
Therefore, even if the operation of the power-assisted steering system is normal, the diagnostic apparatus erroneously determines that the power-assisted direction is abnormal and a fail-safe facility is activated on the basis of this erroneous determination so that the power-assisted steering is stopped.
Although this problem would be solved to some degree by setting a sensitivity in an abnormal criterion to a duller direction, this causes a delay in the abnormality determination due to a reduction in a sensitivity when the abnormality occurs in the power-assisted steering system in a case where the steering torque is large and the absolute value of the normal/reverse drive current is large.
It is, hence, an object of the present invention to provide improved diagnostic apparatus and method for a motor driven power-assisted steering system of the vehicle which can reduce an erroneous determination of abnormality due to an influence of the noises included in the normal/reverse drive current even if the steering torque is small and the current value of the normal/reverse drive current supplied to the motor is small without reduction in the sensitivity in the abnormality determination when the power-assisted steering operation is required.
The above-described object can be achieved by a diagnostic apparatus for a motor driven power-assisted steering system of an automotive vehicle, comprising: a control section that regulates a magnitude of a drive torque and a direction of the drive torque to be applied to an electric motor in a form of a normal/reverse drive torque command current; a drive current detection circuit to detect a normal/reverse drive current to be supplied to the motor; an abnormality determining section that detects a state in which the detected normal/reverse drive current indicates an opposite to a direction of the normal/reverse drive torque command current and determines whether the state is in excess of a predetermined abnormality determination criterion so as to determine an occurrence in abnormality of the power-assisted steering system, the abnormality determining section comprising a sensitivity reducing section that varies a sensitivity of the abnormality determination criterion in a decrease direction as an absolute value of the detected normal/reverse drive current becomes smaller.
The above-described object can also be achieved by providing a diagnostic method for a motor driven power-assisted steering system of an automotive vehicle, comprising: regulating a magnitude of a drive torque and a direction of the drive torque to be applied to an electric motor in a form of a normal/reverse drive torque command current: detecting a normal/reverse drive current to be supplied to the motor; detecting a state in which the detected normal/reverse drive current indicates an opposite to a direction of the normal/reverse drive torque command current; determining whether the state is in excess of a predetermined abnormality determination criterion so as to determine an occurrence in abnormality of the power-assisted steering system; and varying a sensitivity of the abnormality determination criterion in a decrease direction as an absolute value of the detected normal/reverse drive current becomes smaller.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.